Users of mainframe computers running a proprietary operating system may have a very large investment in proprietary application software and, further, may be comfortable with using the application software because it has been developed and improved over a period of years, even decades, to achieve a very high degree of reliability and efficiency.
As manufacturers of very fast and powerful commodity processors continue to improve the capabilities of their products, it has become practical to emulate the proprietary operating systems of powerful older computers such that the manufacturers of the older computers can provide new systems which allow the users to continue to use their highly-regarded proprietary software by emulating the older or “legacy” computer and in particular the central processing units of the legacy system. Accordingly, computer system manufacturers are developing such emulator systems for the users of their legacy systems, and the emulation process used by a given system manufacturer is itself subject to ongoing refinement and increases in efficiency and reliability.